One of the most ubiquitously implicated transcription factors in all of carcinogenesis is nuclear factor-κB (NF-κB). NF-κB represents a family of five proteins (p105/50, p100/p52, c-Rel, RelA/p65 and RelB) that affect over 400 genes, many of which are important in the context of cancer cell growth and survival (Perkins, 2007, Nature Rev. Mol. Cell Biol. 8:40-62). Many of these gene activate genes that: promote cancer cell growth; inhibit those mechanisms responsible for cell cycle arrest; and promote resistance to cell death. All NF-κB members contain a Rel homology domain (“RHD”), an approximately 300 amino acid long residue, which is a highly conserved sequence (e.g. SEQ ID NO:1 from AF134870 and see Lopez-Rodriguez et al., 1999, Proc. Natl. Acad. Sci. 96:7214-7219) near their N terminus of the RHD. This domain is the site that mediates binding to DNA, dimerization with other NF-κB subunits, and nuclear localization. SEQ ID NO: 1 is a human sequence as follows:
LSQLTTDNKGNSKAGNGTLENQKGTGVKKSPMLCGQYPVKSEGKELKIVV QPETQHRARYLTEGSRGSVKDRTQQGFPTVKLEGHNEPVVLQVFVGNDSG RVKPHGFYQACRVTGRNTTPCKEVDIEGTTVIEVGLDPSNNMTLAVDCVG ILKLRNADVEARIGIAGSKKKSTRARLVFRVNIMRKDGSTLTLQTPSSPI LCTQPAGVPEILKKSLHSCSVKGEEEVFLIGKNFLKGTKVIFQENVSDEN SWKSEAEIDMELFHQNHLIVKVPPYHDQHITLPVSVGIYVVTNAGRSHDV QPFTYTPD.
In normal cells, following an activation signal from the surface of the cell, NF-κB subunits translocate to the nucleus where they exert their effect on gene transcription by binding to DNA. The ability to transactivate specific genes in DNA is absolutely dependent on the ability of the NF-κB subunits to enter the nucleus, the site of all DNA replication and transcription.
There are at least three known NF-κB activation pathways: (1) the canonical or classical pathway, which is an IκB dependent pathway activated by extracellular signals such as TNFα, IL-1 and LPS; (2) the non-canonical or alternative pathway, which is an IκB independent pathway activated by CD40/CD40L interaction, and (3) the atypical pathway, which is stimulated by various signals including genotoxic stress, hypoxia and ROS (Perkins, 2007, Nature Rev. Mol. Cell Biol. 8:40-62). Within the classical pathway, NF-κB transcription factors are sequestered in the cytoplasm in their inactive state by the IκB family of inhibitory proteins (IκBa, IκBβ, IκBe, p105/κ and p100/d). Upon an activation signal, IκB kinase (IKK) phosphorylates IκB, rendering it a substrate for ubiquitination and subsequent proteosome mediated degradation. Removal of the IκB allows for nuclear translocation of the NF-κB complex, and activation of its target genes (Perkins, 2007, Nature Rev. Mol. Cell Biol. 8:40-62).
In the alternative pathway, IKK directly phosphorylates p100 which in turn induces the processing of p100 to p52, which is then translocated to the nucleus with subsequent activation of the target genes. The atypical pathway can lead to NF-κB activation in a IKK independent way (hypoxia and ROS activate Tyr kinase) or an IKK dependent way (genotoxic stress). Over expression and constitutive activation of NF-κB is thought to be one of the central events leading to cancer. This biology, first described in normal lymphocytes, is thought to play a pivotal role in the formation of lymphomas.
Identifying pharmacologic strategies to inhibit the activation of target NF-κB genes has been a major pursuit for cancer research laboratories over the past 2 decades.
Over the years, select agents indirectly affecting NF-κB biology have been identified, as discussed below. These agents affect NF-κB biology by inhibiting IκB kinase, which inhibits the phosphorylation and subsequent degradation of IκB, or by inhibiting the proteasome, and thus the proteolytic degradation of IκB.
NF-κB promotes the dysregulated growth and survival of many cancers, including most lymphomas. Efforts to inhibit NF-κB over the years have been fraught with many challenges, not the least of which has been the development of relatively NF-κB non-specific agents. One such example is bortezomib (Velcade), a proteasome inhibitor touted as an NF-κB inhibitor which has been approved for the treatment of myeloma and mantle cell lymphoma. While bortezomib inhibits the degradation of IκB, it also affects more than 90% of the protein turnover in the cell, and thus affects virtually every important cellular process known. Clinically, while effective, the drug is neurotoxic and is associated with an irreversible painful neuropathy. Clearly, more specific NF-κB inhibitors are needed. To date, direct binding of NQBS to the target protein (p65 and p50) has not yet been described within published literature and therefore represents a novel mode of action for a drug that inhibits NF-κB pathway.